This invention is directed to providing power from an energy storage device to a utility network.
Problems in a utility network, or xe2x80x9cfaultsxe2x80x9d, (such as network device failures) can affect how power from an energy storage device is distributed throughout the network. In particular, faults tend to drain energy from the storage device, leaving less energy for distribution throughout other areas of the network and for recovering from voltage xe2x80x9csagsxe2x80x9d resulting from the fault.
When a fault occurs in a utility network, momentary voltage depressions are experienced, which may result in voltage collapse or voltage instability on the network. To better understand the dynamics of a fault on the network, the sequence of events on the network due to a 3-phase fault will now be described. Referring to FIG. 1, assume that the fault occurs on a portion of a network 4 remote from segment 1. Segment 1 lies between a substation 2 and a switching station 3. Referring to FIG. 2, a voltage profile as a function of time at substation 2 due to the fault is shown. In this particular case, the voltage drops from a nominal 115 kV to about 90 kV. If the fault were to occur more closely to segment 1 or on segment 1 itself, the drop in voltage would be more severe, making the voltage approach zero.
In general, such a fault appears as an extremely large load materializing instantly on the utility network. In response to the appearance of this load, the network attempts to deliver a large amount of current to the load (i.e., the fault). Detector circuits associated with circuit breakers on the network detect the overcurrent situation immediately (within a few milliseconds). Activation signals from the detector circuits are sent to protective relays which initiate opening of the circuit. The mechanical nature of the relays generally requires 3 to 6 cycles (i.e., up to 100 milliseconds) to open. When the breakers open, the fault is cleared. However, opening of the breakers triggers a sequence of cascading events which, in the extreme, can cause the voltage on a portion of the utility network to collapse. Specifically, when the breakers open, the voltage is still low (i.e., almost zero) and, because a portion of the transmission network has, in effect, been removed, the impedance of the network dramatically increases causing the appearance of an artificially high load. In this state, the voltage is depressed and the current serving the load increases sharply. The sharp increase in current generates enormous losses in the network. In some cases, the voltage on the network may not return to normal, causing long-term voltage depression and the possible voltage collapse of the entire network. The potential for these voltage instability problems is further exacerbated as load requirements on the network increase.
One approach for addressing the foregoing problem is to construct additional transmission lines, thereby negating effects of the high losses and sharp increase in current flow caused by the opening of the circuit breakers. However, providing such additional lines is expensive and, in certain, settings, extremely difficult.
In general, in one aspect, the invention relates to providing power from an energy storage device to a utility network. The invention features determining whether a fault on the utility network comprises a near fault or a far fault relative to the energy-storage device, and supplying power to the utility network based on whether the fault is a near fault or a far fault.
Among the advantages of the invention may be one or more of the following. Faults that occur near to the energy storage device absorb large amounts of real power. Most of the real power from the energy storage device is thus drawn into an energy sink created by the fault. This is known as xe2x80x9cfeeding the faultxe2x80x9d. By contrast, faults that occur far from the energy storage device do not act as energy sinks, at least not to the same degree as near faults. By supplying power based on whether the fault is near or far, it is possible to adjust the power so that the energy storage device is not depleted unnecessarily. For example, if the fault is a near fault, reactive power may be supplied to the utility network (since real power would go to feed the fault, it is not supplied). If the fault is a far fault, both real and reactive power may be supplied to compensate for the far fault.
This aspect of the invention may include one or more of the following features/functions. The energy storage device may be a current storage device, such as a superconducting magnet. Both real and reactive power may be supplied to the utility network if the fault is a far fault. Reactive power may be supplied to the utility network if the fault is a near fault.
If a near fault is detected, real power may be supplied to the utility network after the near fault is at least partially cleared. In this case, only reactive power is supplied to the utility network before the near fault is at least partially cleared.
The fault is determined to be a near fault or a far fault by detecting a voltage drop in the utility network. If the voltage drops below a predetermined level, the fault is classified as a near fault. For example, if the voltage drops below a first percentage of the nominal voltage of the utility network, the fault is classified as a near fault. If the voltage drop is not below a predetermined level, the fault is classified as a far fault. For example, if the voltage drops to between the first percentage and a second percentage of the nominal voltage of the utility network, the fault is classified as a far fault.
In general, in another aspect, the invention features a system which includes an energy storage device and an inverter which provides energy from the energy storage device to a utility network. A controller controls the inverter to provide real and/or reactive power to the utility network based on a detected condition (e.g., a near or far fault) in the utility network.